Cobalt thiocyanate relative humidity indicators



composition for Patented Jan. 25, 1949 COBALTVTHIOCYANATE RELATIVE HUMID- ITY INDICATORS Paul Davison Chemical Corporation,

Bell Davis, Baltimore, Md., assignor to The Baltimore,

No Drawing. Application February '7, 1945, Serial No. 576,725

' .2 Claims. (01. 252-408) This invention relates to indicators and more particularly has reference to compositions for indicating the presence and quantity of moisture in a gaseous atmosphere.

An object of this invention is to provide a indicating the presence and quantity of moisture in a gaseous atmosphere within predeterminedlimits.

Another object of this invention is to provide a composition capable of exhibiting a definite color break when exposed to a gaseous atmosphere having a relativehumidity ranging from about 20.to 40%. I

A further object of this invention is to provide a moistureindicator formed of cobalt thiocyanate associated with an adsorbent carrier such assilica gel.

Still another object of this invention is to provide a moisture indicator formed of an adsorbent material such as silica gel having Co(S CN)2 incorporated in the pores thereof, in which the adsorbent characteristics of the carrier, its moisture content, the color changing characteristics of the Co(SCN)2 and its concentration are correlated to produce a definite break in the color of the indicator when exposed to a gaseous atmosphere having a predetermined relative humidity.

It is also an object of a method of making a indicator.

Other objects description:

In accordance with the present invention it has been found that by treating an adsorbent carrier such as silica gel with an aqueous solution of C(SCN)2 and drying the so treated carrier, an indicator gel is obtained which has a definite color and which upon exposure to a gaseous atmosphere such as air having a prethis invention to provide color changing moisture will appear from the following determined relative humidity will change its v color.

It has also been found, in accordance with the present invention, that by correlating the adsorptive characteristics of the carrier, its final moisture content as fixed by the temperatures at which the impregnated carrier is dried, or dried and activated, and the concentration of cobalt thiocyanate in the carrier, the color of the resulting indicator before exposure to a humid atmosphere and also the color of the indicator after exposure to an atmosphere having a predetermined relative humidity can be adjusted or controlled. l

The present invention is directed principally eniploye'd. The silica gel'in to moisture indicators formed of a carrier such as silica gel impregnated with cobalt thiocyanate and having a definite color change or .break when exposed -'-to a gaseous atmosphere such as .air having a relative humidity in the range of 20 to 40%. V

In preparing an indicator according to the present invention, a standard silica gel made by washing an acid reacting hydrogel with an acidic wash water at a-temperature'of about 150 F., drying and thenactivating at a temperature of about 850 F. and sized to about 6 to 16 mesh, was the activated form exposing it to a humid atv high relative humidity to saturate the gelwith moisture to such an extent that upon-contacting it with a'liquid it would not disintegrate as would be the case if freshly actiwas first treated by mosphere having a 'va'ted gel were contacted with a liquid.

After saturation with moisture to an approximate total volatile content of about 30%, the silica gel was impregnated with an aqueous solution of cobalt thiocyanate. It has been found that by first saturating the gel with moisture to the extent indicated, wetting of the gel with the Co(SCN)2 solution can be effected by using about ml. of solution per kilogram of gel. Of course, if necessary, more of the solution can be employed. The cobalt thiocyanate solution used was of such concentration and the quantity of solution per kilogram of gel was such that the gel had a C0(SCN)2 content of about on a dry basis.

By drying the silica gel having a CO(SCN)2 content of about at a temperature of about 250 F., an indicator was Munsell color value of about 1.5 PB 3/ 14. This indicator was found to be particularly suitable for the indication of the presence of moisture in a gaseous atmosphere such as air when the atmosphere had a relative humidity ranging between 20 and. 40%. For instance, when this indicator gel was exposed to an atmosphere having a relative humidity of about 20%, the color of the indicator changed to about 10.0 PB 5/8 M. C. V. When the same indicator gel was exposed to an atmosphere having a relative humidity of about 40%, the color changed to about 7.5 RP 6/ 8 M. C. V.

By varying the concentration of the cobalt thiocyanate in the gel, the characteristic-s of the gel or the temperature at which the impregnated gel was dried or activated, a product was obtained which responded by changes in color when rier of the chromatic with the alkaline V 7 number uof-microscopic-pores throughout the body exposed to atmospheres having difierent relative humidities. V g

* In the drying and activating of the abovementioned gels, the same were held at the temperatures mentioned for about three hours. 7 7

While silica gel has been described as thecarmay be used as described in my cope'nding application, Serial No. 576,726 (nowrabandonedlfi filed of even. date herewith. Such porous glass is made from a glass containing from 60 450.82%

S102, 20 to 35% B203 and 5 to 10% alkali-oxide heat treated at a temperature'of 1525-01'600? .C. to I cause the glass to be separated :into two 'p ases, one of which is rich in boric oxide and alkali oxide and is soluble in acids, and .the other of which is very rich in silica and substantially insoluble in acids. By treating the so heat treated glass with an acid such as hydrochloride, nitric or sulphuric acids maintained at a temperature at or near the boiling pointthereof, the acid reacts phase of the glass forming a thereof. This porous glass may be used in place of the silica gelhereinbefore mentioned. 7

The values given for the variou-s'colors in the foregoing description refer to thesystem of color notation devised -by'A..H.;Munse11 and published by Munsell Color :Company, Inc., Maryland, as :the

Baltimore,

From the foregoing description, it tvill be appreciatedthat the present invention provides anindicator gel particularly suitable for the indication of the relative It will also be appreciated that the present invention provides methods of preparing and utilizing indicator gels .ihaving color breaks when exposed to atmospheres. :of different relative 'humidities. Y 7

chemical; a porous glass Munsell-Book of Color, abridged 7 edition, revised 1942. r

humidity of an atmosphere. :1

activated silica gel with about 4%,

I claimi 1. A method of preparing a quantitative relative humidity indicator comprising impregnating on a dry basis, of cobalt thiocyanate, anddryingthe impregnated silica gel at:a temperature of about 259 F., said indicator changinglcolor when exposedito an atmosphere having a relative huwith about on cyanate, said indicator changing color when exmidityrof 20-40%. V

2. A quantitative relative humidity indicator ccnsisting essentially of silica gel impregnated a dry basis, of cobalt thiopriced to -an:atmosphere havinga relative humidity ranging from '29 to 40% and being prepared 7 bythe method ofclaim 1.

Engineer, July 14, 1939,

7 PAUL BELL DAVIS.

REFERENCES CITED 'Thefcllowing references are of record in the of this patent:

UNITED STATES .EATENTS OTHER REFERENCES 7 The Chemical Trade page 3LT Journal and Chemical 7 

